The present invention relates to frequency tracking arrangements. In particular although not exclusively the invention is concerned with frequency tracking arrangements by means of which the cut-off frequency of a continuous-time filter and/or the delay of one or more delay lines may be controlled in dependence upon the frequency of a reference frequency input signal.
Such an arrangement may be used, for example, in the retrieval of data from an optical disk data carrier, where data is stored as a predetermined number of bits per unit length of data storage path. When reading such data carriers, the disk must rotate faster when data is being read from near the center of the disk, or the data rate is slower, than when data is being read from near the periphery of the disk. A continuous frequency tracking/filter cut-off arrangement would allow data to be read at the different rates or while the disk is being speeded up or slowed down.
Automatic adjustment of a filter characteristic is known, for example, from prior patent specification GB 2194402, in which the time constant of a filter circuit is controlled by a frequency tracking loop utilizing a voltage-controlled oscillator. The control of the delay introduced by a delay line is described in GB 2130845, where the phase shift corresponding to that delay is compared with a fixed 90.degree. phase shift of an input signal in a closed loop system.